1. Field of the Invention
This invention relates to steel cords used as a reinforcing member for rubber articles such as pneumatic tires, industrial belts and the like. More particularly it relates to a steel cord having improved bending rigidity.
2. Description of the Related Art
In order to improve the steering performance and stability during the running of the vehicle, it is advantageous to increase a cornering force generated in a direction perpendicular to the running direction of the vehicle per a constant steering input. In order to increase the cornering force, it is required to increase the lateral slipping deformation of a land portion in a tread generated at a ground contact area of the tread during rotation of the tire. The quantity of such lateral slipping deformation is influenced by a deformation of a belt supporting the land portion of the tread or a deformation created in a belt layer shown in FIG. 1a in a plane of the belt layer or along the plane of the belt layer shown in FIG. 1b (hereinafter referred to as in-plane beginning deformation). That is, in order to produce a large cornering force, it is favorable to control the in-plane bending deformation of the belt and hence it is required to increase the ability to resist the in-plane bending deformation (hereinafter referred to as in-plane bending rigidity).
On the other hand, in order to improve the ground contacting property between the tire tread and the road surface, it is effective to sufficiently ensure a ground contacting area even against some irregularity of the road surface. For this purpose, it is required to decrease resistance to a deformation created in a direction perpendicular to the plane of the belt (hereinafter referred to as out-of-plane bending rigidity).
For achievement of bending rigidity required in the belt of the tire, therefore, it is required to rationalize bending ridigities in different in-plane and out-of-plane directions, respectively. These bending rigidities are influenced by the properties of steel cords used as a reinforcement for the belt. That is, the bending rigidity of the belt can be increased by using a steel cord having a high bending rigidity or by increasing an end count of steel cords in the belt.
On the other hand, a single twisting cord of 1.times.5 structure shown in FIG. 2a or a layer twisting cord of 2+6 structure shown in FIG. 2b is generally used as the steel cord used in the belt. In order to increase of the bending rigidity of these cords, it is effective to increase a diameter of a steel filament constituting the cord.
However, the structure of the above conventional steel cord is considered to be a rotating body centered around an axis of the cord, so that the structure is substantially uniform even in any directions crossing with the axis of the cord. As a result, the increase of the bending rigidity based on the increase of filament diameter acts on both of the in-plane bending rigidity and out-of-plane bending rigidity. That is, in the above conventional cord structures, there is a conflicting relation between the increase of in-plane bending rigidity and the decrease of out-of-plane bending rigidity. Therefore, the establishment of these requirements is difficult in the steel cords for the reinforcement of the belt.
As a solution for this task, there are proposed the following steel cords in which the bending rigidities of the cord are different in the bending directions.
For example, there are proposed a single steel filament having an ellipsoidal shape in section as shown in FIG. 3a and a cord obtained by twisting steel filaments of ellipsoidal shape in section as shown in FIG. 3b. In this case, it is difficult to conduct drawing at a high reduction of area while holding the ellipsoidal shape in section, so that there is a problem that a high tensile strength can not be obtained. Furthermore, the cord obtained by twisting of such flattened filaments has a problem in that it is difficult to twist these flattened filaments while setting the major axis (or minor axis) direction of the ellipse in each flattened filament.
Furthermore, the cord of a single twisting structure is flattened as shown in FIG. 3c, or the cord of layer twisting structure is rendered into the ellipsoidal shape in section by using two strands as a core in the cord as shown in FIG. 3d. In this case, the forming shapes of steel filaments constituting the cord differ in accordance with the position of the steel filament. That is, the curvature of the helically formed steel filament differs in the longitudinal direction of the filament. When the cord is bent, the movement of the filament followed to the bending hardly occurs and hence not only the bending rigidity in the major axis direction at the cord section but also the bending rigidity in a direction perpendicular thereto (the minor axis direction at the cord section) become high.
Moreover, there is a cord in which four steel filaments are arranged side by side and helically wrapped with a filament as shown in FIG. 3e. In this case, rigidity can largely be differed in accordance with the bending direction of the cord. However, in order to maintain the side-by-side state of the filaments and enhance the bending direction in the side-by-side direction, it is required to increase the clamping force of the wrapping filament, whereby the pressure between the filaments contacting each other in line becomes high and hence the fatigue property when being repeatedly subjected to bending input is considerably degraded. Also, it is technically difficult to increase the clamping force of the wrapping filament while maintaining the side-by-side state of the filaments.
On the other hand, JP-B-49-47416 proposes a metal cord formed by matching two metallic wires of S lay with two metallic wires of Z lay in longitudinal direction thereof and wrapping them with another wiring body. This cord is formed with a protruding portion in section for improving productivity and the adhesion property to rubber. However, since the cord is the combination of two kinds of two twisted metallic wires, a portion having a non-flattened shape at section is existent in the longitudinal direction of the cord, so that the bending rigidity in the longitudinal direction of the cord is discontinuous and hence there is a large problem in the fatigue property.